The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, the widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. The catalytic reforming unit, therefore, must operate more efficiently at higher severity in order to meet these increasing aromatics and gasoline-octane needs. This trend creates a need for more effective reforming catalysts for application in new and existing process units.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction, thus producing a higher yield of aromatics and a lower output of fuel gas, while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates, as well as naphthas, has been disclosed. The sensitivity to water during regeneration of reforming catalysts in general, and of these selective catalysts in particular, also is known.
Catalytic processes for the conversion of hydrocarbons are well known and extensively used. Invariably, the catalysts used in these processes become deactivated for one or more reasons. Where the accumulation of coke deposits causes the deactivation, regeneration of the catalyst to remove coke deposits restores the activity of the catalyst. Coke is normally removed from the catalyst by contact of the coke-containing catalyst at high temperature with an oxygen-containing gas to combust and remove the coke. This regeneration can be carried out in situ or the catalyst may be removed from the reactor where the hydrocarbon conversion takes place and transported to a separate regeneration zone for coke removal.
Many hydrocarbon conversion processes, such as naphtha reforming processes, employ two or more separate reactors through which a hydrocarbon feed stream passes in series. Typically, each reactor contains a bed of catalyst. The hydrocarbon feed stream passes from one reactor to the next reactor in series through conduits. In naphtha reforming, the hydrocarbon conversion reactions are endothermic, and, therefore, a heater is typically located upstream of each reactor in order to provide the necessary heat of reaction to the hydrocarbon feed stream. In addition, an indirect heat exchanger is typically located downstream of the last reactor in the series, in order to conserve energy by recovering heat from the effluent stream and transferring that heat to the feed stream upstream of the first heater.
Many reforming processes employ a compressor to compress the hydrogen-rich net gas from the reforming process from the relatively low pressure of the reforming reactors to a relatively high pressure that is required for the net gas to be employed in other downstream processing units. Also, many reforming processes employ a chiller to cool the net gas in order to condense and recover the light hydrocarbons that are present in the net gas. The purified net gas is then employed in other downstream processing units. Some reforming processes employ both a net gas compressor and a net gas chiller. In these processes, the combination of the elevated pressure and the reduced temperature achieve a higher purity of the net gas than either elevated pressure or reduced temperature alone.
In hydrocarbon conversion processes employing two or more reactors, arrangements for regenerating the hydrocarbon conversion catalyst in situ semicontinuously are well known. In semi-continuous regeneration, all of the reactors are periodically taken out of service and are regenerated by passing the oxygen-containing gas through the reactors in series. The oxygen-containing gas passes from one reactor to the next reactor through the heaters, heat exchangers, and conduits through which the hydrocarbon-feed stream passes when hydrocarbon conversion takes place. Coke combustion is controlled by recycling the oxygen-containing gas, by adding a small stream of make-up air to replace oxygen consumed in the combustion of coke, and by venting off a small amount of flue gas containing the by-products of coke combustion to allow for the addition of the make-up air. While coke burning progresses from one reactor to the next reactor, the steady addition of make-up gas and the venting of flue gas establishes a steady state condition that produces a nearly constant concentration of water in the circulating regeneration gases. This steady state concentration of water in the circulating gases is higher where, in addition, the circulating regeneration gases are contacted with and then separated from an aqueous solution. One example in the prior art of the desirability of contacting the circulating gases with an aqueous solution arises where the circulating regeneration gases contain a halogen-containing compound such as hydrogen chloride and the regeneration gases are contacted with a basic, aqueous solution in order to neutralize the hydrogen chloride. The concentration of water in the circulating gases is generally higher when there is contacting with an aqueous solution than when there is not contacting. This is because the water concentration of the regeneration gases, after having been separated from the aqueous solution, is the saturation concentration of water at the conditions of the separation, which is generally much higher than the steady state concentration arising from the water of coke combustion alone.
One problem associated with coke combustion is catalyst deactivation. The combination of temperature, water vapor, and exposure time determines the useful life of the catalyst. Exposure of high surface area catalyst to high temperatures for prolonged periods of time will create an amorphous material having a reduced surface area which in turn lowers the activity of the catalyst. In contrast to catalyst deactivation by coke deposition, deactivation of this type is permanent, rendering the catalyst unusable. When moisture is present--water is a by-product of the coke combustion--the deactivating effects of high temperature exposure are compounded.
Various methods have been proposed in the prior art for reducing the water present during regeneration of catalysts, but these methods require the use of expensive additional drying equipment, such as large beds of desiccant. These beds of desiccant are expensive both to construct and to operate, because of the relatively large volumetric flow rate of circulating recycle gas flowing through the beds, because of the water that is present as a result of contacting the regeneration gases with an aqueous solution, and because of the water that is produced as a by-product of coke combustion. Also, during typical in-situ regenerations of the prior art, the net gas compressor and the net gas chiller, where present, generally sit idle and are not employed in reducing the water present during regeneration.
Therefore, there is a need for a method for reducing the water content during the regeneration of a catalyst in a hydrocarbon conversion unit that does not require the use of additional expensive equipment and maximizes the use of existing process equipment.